1. Field of the Invention
The present invention relates to methods and devices for executing a synchronous detection of a multiplex signal as an input signal composed of a plurality of carrier waves having a different frequency in order to obtain an amplitude value of each of the carrier waves.
2. Description of the Related Art
In order to correctly receive a multiplex signal with FDM (Frequency Division Multiplexing) or OFDM (Orthogonal Frequency Division Multiplexing), which is composed of a plurality of carrier waves, transmitted from a transmitter, a receiver must have a synchronous detection device. The synchronous detection device synchronously detects each of the carrier waves having a different frequency from the received multiplex signals.
For example, Japanese patent laid open publication JP 2005-102129 has proposed a conventional synchronous detection device which sequentially calculates a moving-average value of received signals every half (or quarter) period of a carrier wave having the maximum frequency in a plurality of carrier waves as a detection target. The conventional synchronous detection device adds the moving average value per carrier wave in the front half of the period of this carrier wave, and subtracts the moving average value per carrier wave in the latter half of this carrier wave in order to execute the synchronous detection of each of the carrier waves.
The conventional synchronous detection device has a time analogue to digital converter (TAD) equipped with a pulse delay circuit composed of a plurality of delay units connected in series in order to calculate such a moving average value of each of received carrier waves. This structure of the conventional synchronous detection device makes it possible to execute the synchronous detection of a plurality of the received carrier waves simultaneously without using any analogue circuits such as an operational amplifier and an analogue filter.
However, the conventional synchronous detection device described above can execute the synchronous detection only for a multiplex signal obtained by synthesizing carrier waves having a frequency which is a power of two times the frequency of a reference carrier wave (i.e. 2nfc where n=0, 1, 2 . . . , and fc is the carrier frequency) because the reference carrier wave has the minimum frequency in the received carrier waves. That is, it is difficult for the conventional synchronous detection device to execute the synchronous detection of a multiplex signal obtained by synthesizing carrier waves having an integer multiple (1, 2, 3, . . . , n times) of the frequency of the reference carrier wave.
That is, if the conventional synchronous detection device previously described executes the synchronous detection of such a type of the multiplex signal having a integer frequency multiple (1, 2, 3, . . . , n times) of the frequency of the reference carrier wave, the obtained detection result contains carrier wave components having an odd frequency multiple of the frequency of this carrier wave. The conventional synchronous detection device provides a correct result only when the multiplex signal obtained by synthesizing carrier waves has a frequency which is the power of two times the frequency of the reference carrier wave. This problem limits the use and application of the conventional synchronous detection device.
Further, such a multiplex signal obtained by synthesizing carrier waves having various frequencies which are the power of two times of the frequency of the reference carrier wave needs to use a large frequency band when compared with a frequency band of a multiplex signal obtained by synthesizing carrier waves having an integer multiple frequency of the reference carrier wave only when they has the same number of the carrier waves.
As described above, the conventional synchronous detection device decreases the efficiency in use of the frequency band, and the structure of the conventional synchronous detection device limits the application field thereof. Further, the structure of the conventional synchronous detection device is required to have a large circuit size because of using a wide frequency band, and increases the cost of a communication system. Still further, the conventional synchronous detection device has a low detection quality because noises are easily suspended on carrier waves by using such a wide frequency band.